Deep Fear aka Pure Dead Frozen

the long version

As usual, the Strega-Borgia family can't seem to get a break: They've just welcomed a newborn boy Strega-Borgia into the world, but before they can give him a name, he's snatched and replaced with a demon baby. Mrs. McLachlan, the super-efficient (and supernatural) nanny is back, safe and sound--but so is the demon Isagoth, and he hasn't forgotten what he wants from the Strega-Borgias. And evil Uncle Lucifer has resurfaced, but this time he has the added ammunition of a pact with S'tan the First Minister of the Headen Executive--otherwise known as His Imperial Inflammableness, his S'tainless Steeliness, and the Arch-Fiend. And S'tan himself is desperate to find the Chronostone, because. . . well, to put it bluntly, Hades is starting to freeze over, and without the Chronostone, he may never regain his power.The family's got to be ready--because this will be the final battle between good and evil, the battle where winner takes all. And they're going to need all the help they can get. . . .From the Hardcover edition.